We hypothesized that left-colon water exchange colonoscopy (LWE) may have the similar effect regarding the maximal pain score and mean pain score in unsedated patients compared with whole-colon water exchange colonoscopy (WWE). At the mean time, LWE may reduce the insertion time compared with WWE. Here we performed a prospective, randomized controlled study to investigate the efficacies of whole-colon water exchange, left-colon water exchange and air methods of colonoscopy on pain score and insertion time in unsedated outpatients.